CA1069774A - Spray application of aqueous paints by controlling temperature and relative humidity of air in the spray zone - Google Patents

Abstract

AN IMPROVED PROCESS FOR THE SPRAY APPLICATION
OF AQUEOUS PAINTS BY CONTROLLING THE TEMPERATURE
OF THE AIR IN THE PAINT SPRAY ZONE

ABSTRACT OF THE DISCLOSURE

The process is directed towards an improvement in the application of an aqueous paint to a substrate by air spraying the paint with a spray gun utilizing atomizing air at a flow rate of about 5-40 cubic feet per minute at an air pressure Or 30-95 pounds per square inch at the spray gun and paint flow rate of about 5-30 ounces per minute, wherein the air atomized paint is applied to the substrate and baked to form a uniform finish on the substrate;
the improvement that is used with this process comprises spraying atomized paint in an atmosphere that is controlled at a temperature of about 15-40°C. and at such a relative humidity that the air has a drive force value with the range of 0.002 to 0.004 humidity units (pounds of water/
pound of dry air) which is constantly maintained at a set value within this range to initially dry the paint at a uniform rate to form a finish that has a uniform appearance after being baked;
the improved process is particularly useful for applying an exterior aqueous finish on automobile and truck bodies to form a high quality finish of uniform color, hue and appearance.

Description

3~

BACKGROU~nD OF THE INVENTION
Thiæ invention relate~ to an lmproved proce~ ror applying aqueou~ coatlng composition~ and in parti¢ular t~ an improved proces~ ~or the appli¢ation o~ thermo-setting acrylic enamel coating compoæition~ to prov~de finiæhe~ having a unifoFm appearance.
Thermosetting acrylic en~mels are well known in the ~rt as ~hown by Fraizer et al, U.S. Patent 2,681,897, i~su~d 3une 22, 1954; Vasta, U.S~ Patent 3,338,~60, is~ued Augu~t 29, 1967; Fi~k et al., U.S, P~tent 3,365,414, i~sued ~anuary 23, 1968; Vasta, U.S. Patent 3~6 æ ,651, ls~ued November 23, 1971; Parker, U.S. Patent 3,637,546, issued January 25, 1972. These enamels are æolvent based sy~tems and the spray application of the~e enamel~ has not posed a problem~ However, aqueou~ ba~ed acryl~c enamels as lllu~trated by Ta~t, U.S. Patent 3,661,827, is~ued May 9, 1972, prsvide problem~ ln drying Or the rinish during and a~ter applicst~on. If the appllcation i3 on a very humid day, the paint doee not dry and run8 12 and 8ag~, and conversely~ on very hot, dry days, the paint dries exce~sively as it is sprayed onto the panel, thereby giving a poor appearance. Also, variation~ ln the humidity whlch do not cau~e di~iculties in drying cAuse change~ in the appearance o~ the paint ~ilm.
In particular, paints containing metallic flakeæ, such as aluminum ~lakes, the appearance change~ æubstantially under varying humidity conditionæ.
m e improved procegs o~ thiæ invention provides ~or in~tial drying condition~ in which the air iæ
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under controlled temperature and humidity conditions which re~ults in ~inishe~ that consistently have an excellent and uniform appearance without using conven-tional air condltioning, i.~., cooling of the air to reduce temperature and humidity.

SUMMARY OF T~E INVENTION

m e improved proce~s o~ this invention is directed to the application o~ an aqueous paint to a ~ub-strate by air spraying the paint wlth a ~pray gun utllizing atomizing air at a ~low rate of about 5-40 cubic feet per minute and at an air presæure o~ 30-95 pounds per square inch at the spra~ gun and a paint flow rate o~ 5-30 ounces per minute to form air atomized paint which then 18 applied to the sub~trate and subse-quently baked at 75 to 200 C. to ~orm a ~ini~h on the sub~trate, the improvement used therewith comprises spray-ing the atomized paint in an atmo~phere that is controlled at a temperature o~ about 15-40C. and at ~uch a relative humidity that the air ha~ a driving force value in the range o~ 0.002 to 0.004 humidity units which is const~ntly maintained at a set value within this range, thereby providing a uni~orm atmosphere for initially drying the aqueous paint during ~praying to form a finish that has a uni~orm appear~nce after being baked.

BRIEF DESCRIPTION OF '~ DRAWING

The Flgure illu~trates a schematic diagram for automatically ad~usting the temperature and humidity of air in a paint spray booth.

~ ~774 DESCRIPTION OF THE INVENTION
In a conventional process ~or apply~ng aqueou~
paints to a sub~trate by air Epr~ying the paint, a spray gun i8 utillzed which atomize~ the paint to a fine spray to apply the paint to a ~ub~trate. Generally, the atomiz-lng air is utilized at a flow rate of 5-40 cubic Peet per minute and under a pressure o~ 30 to 95 pounds per ~quare inch at the spray gun. The flow rate o~ palnt passing through the gun i8 about 5-30 ounce~ per minute.
m e sprAy gun form~ a unirorm cone of air atomized paint which 1~ applied to the substrate and ~ubsequently the ~ubstrate i~ baked at about 75-200nC. to provide a high quality ~inish. One pre~erred baking cycle for aqueous thermosetting acrylic enamels comprlsing a pre-bake ak about 75-95C. for about 5-30 minutes and then a final bake at about 125-200C. to ~orm the flnish.
The resulting ~iniBh i8 about 0.5-3.5 mils in thickne~s and prePerably, 1.0-2.5 mils in thickness. Generally, the~e ~lnishes can be rubbed or po113hed in accordance with conventional technique~ to improve glOB8 .
This conventional spray application process pro-vides adequate finishes under ideal drying conditions, for example, 25C. and 50% relati~e humidity. However, problems arlse under high humidity conditions, ~or example, relative humidity of 90% and above allo~æ the aqueou~
finish to dry at a very ~low rate and aæ a result, the ~lnish has a poor appearance caused by sagging and running o~ the paint and also a color di~erence is often noted, and conversely under low relative humidity conditions such as 0-10% relative humidity, the atomized paint dries excessively while being ~prayed onto the sub~trate and results in a gritty finish which has a poor appearance and also poor phy~ical properties. Even under condition~ which are conducive to drying, the ~inish produced from the aqueouæ paint has a di~ferent hue or shade and surrace texture at various levels of relative humldity; in paxticular, this i8 most pronounced with paint containing metall~c ~lake pigments.
me improved process of this invention economi-cally provides a controlled air atmosphere for the initlal drying of the finish which reæults ln the uni~orm drying o~ the finish which is extremely important ~or the appearance of this finish. Thi~ is accomplished by controlling the air atmosphere at about 15-40C. and at such a relative humldity that the ~ir has a driving ~orce ~alue in the range of 0.002 to 0.004 which is constantly mainta~n~d at a set value within this range.
Pre~erably, the air has a driving force value o~ 0.003 humidity units.
The driving force value of the air is the humidity of the air corresponding to the adiabatlc sat-uration temperature expre~sed in pounds of water per pound o~ dry air less the humidity o~ the ambient air also exprea~ed in pounds of water per pound o~ dry air. mese humidity values are determlned with a standard humidity-temperature chart, ~or example~ shown in John H. Perry's Chemical Engineer~s Handbook, 4th Edition (McGra~ Hill Chemical Engineering Series)~

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~(~6977'4 me humidlty of amblent air at a low relative humidity may be ad~usted by the addition of ~team or water or both. By proper choice o~ amount o~ ~team and water, the air temperature can be ad~u$ted. Ir the tem~erature o~ the ambient air ls exceptionally high and the relative humidity low, water may ~e sprayed into the air to incre&se the humidity and also to lower the air temperature. If the ambient humid~t~ is high, the air can be heated to ~uch a temperature that the de~ired driving ~orce ~or dryin~ of the palnt i~ achieve~.
The spray proce~æ can be further controlled by heating the atomlzlng air from 40 to 150C. whi~h, o~
cour~e, heats the paint on atomi~ation and enhanceæ the drying~ This technlque i~ particularly useful under high relative humid~ty condition~ and increaæe~ the rate o~
drying o~ the paint during application thereby eliminating aagglng and running o~ the paint.
The novel process of this invention is particularly advantageous in that it allows for a uniform spray applica-tion o~ the aqueous based paints under all type~ o~ relativehumidity condition and does not require the manu~acturer to completely alr condition a spray area. mi~ technique reduce~ a manufacturer~s initial costs and manu~acturing costs since air conditioning equipment is not required. It ls ~ubstantially more economical to heat or humidi~y or heat and humldify the air in the ~pray zone to control the evapora-tion than to air condition the entire spray area to a prescribed temperature and humidity. me novel process is particularly useful in the ~pray zones where the paint i8 applied by automatic machines.

16~69~4 Convention~l air conditioning brings air to a fixed ~et of temperature and humidity conditions and u~e~
one or a combination of the following ~tep~ heating, cooling, humidification and dehumidification to achieve these condi-tions and a~ pointed out i~ an expen~ive proceEs. me proce~s o~ this invention only uses heat and humidification and does not attempt to maintain const~nt temperature and humidity conditions as does air conditioning. The procesæ
of this invention maintains a constant dri~ing force value ~o that the evaporation rate of the palnt belng sprayed remains at the æame level to provide finl~he~ with a unl~orm appearance.
me figure illuætrate~ one practical syætem for controlling the temperature and humidity of the air fed into the controlled spray ~one of a spray booth. Out~ide air is pa~sed through a water 3crubber to remove dust particles and clean the air. The air is next pasæed through a ~ilter to remove any residual particles. A
temperature indicator ~nd a relative hum~dity indicator are posltloned in the air llne which are electrically connected to a computer controller which monitors the ~lgnal from the two indicators. The air paæses through a heater and through a humidifier which are electrlcally or pneumatlcally couplea to the computer controller and controlled by the computer controller.
me heater i~ a conventional heat exchanger wherein the air is passed over a heated æurface.
Steam or hot water or another fluld can be u~ed in the heat exchanger and the fluid flow is controlled by a valve that i~ electrically coupled to and controlled by the computer controller.

1()6~774 The humidifier can utilize a water spray or steam to increase the water content in ~he air. The humidifier is coupled to and controlled by the computer controller.
The air is then passed onto the portion of the spray booth ~hat is being automatically controlled. The air passes over a second set of temperature and humidity indicators electrically coupled to the computer controller~
The computer controller receives the signals from khe temp-erature and humidity indicators and utilizes the data to control the heater and humidifier.
The novel process of this invention can be utilized with a variety of aqueous based paints to improve the drying and appearance of the finish. Preferably, the novel process is utilized with aqueous thermosetting acrylic coating compositions and in particular thermosetting acrylic coating compositions in which the film-forming consituents comprise (1) an acrylic polymer selected ~rom the group consisting of (A) a polymer of (a) at least one of styrene and methyl methacrylate, (b) at least one of an alkyl acrylate and an alkyl methacrylate, where the alkyl group has 2-12 carbon atoms, and (c) an ~,~ unsaturated carboxylic acidg and (B) a poly-mer of (a), (b) and (c) and at least one of a hydroxy alkyl acrylate and a hydroxy methacrylate; and

(2) a ~ater-soluble or water-dispersible cross-linking agent.

,~ , ~., 106~7~4 One use~ul aqueous thermosetting acrylic enamel contains the ~ollowing ~ilm-form~ng constituent~:
(1) 60-90% by weight of an acrylic polymer o~
(a) 20-60% by weight of methyl methacrylate or ætyrene or a mixture of methyl methacrylate and ætyrene, (b) 20-40% by weight o~ an &lXyl group having 2-12 carbon atoms in the alkyl group or an alkyl methacrylate having 4-12 carbon atom~ in the alkyl group;
(c) ~-20% by weight of an ~,~-ethylenically unæaturated carboxylic acid such as acrylic acid, methacrylic acid or itaconie acid; and (2) 10-40% by weight o~ a water dispersible or water-soluble cro~æ-linking resin ~uch as an alkylated melamine ~ormaldehyde reæin having 1-4 carbon atom~ in the alkyl group, the compo~ition contains ~uf~icient baæic compound to provide a pH o~ about 6-10.
One pre~erred aqueous thermosetting acrylic enamel coatlng compoæition utilized with the proce~s o~
this lnvention compriæes 10-60% by ~eight of film-~orming conætituents, where-in the ~ilm-~orming conætituents con-3ist eæsentially o~
(1) 60-90% by ~eight, based on the weight o~ the film-~o~min~ constituent~, o~ ~n acrylic polymer having a uniformlty factor o~ at lea~t 75% which consistæ essentiplly o~

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(a) 20-50~ by weight, based on the weight of the acrylic polymer,- of a hard constituent which is either methyl meth-acrylate, or a blend of methyl methacrylate and styrene wherein the styrene comprises up to 40% by weight of the acrylic polymer;
(b) 20-40~ by weight, based on the weight of the acrylic polymer, of a soft acrylic constituent that is either an alkyl acrylate having 2-12 carbon atoms in the alkyl group, an alkyl methacrylate . having ~-12 carbon atoms in the alkyl group, or a mixture of the above alkyl acrylate and alkyl methacrylate;

(c) 4-20% by weight, based on the weight of the acrylic polymer, of a hydroxy-containing constituent which is either a hydroxyalkyl methacrylate or a hydroxyalkyl acrylate or a mixture thereof in which the ? alkyl groups have 2-4 carbon atoms~

(d~ 4-20% by weight, based on the weight of the acrylic polyrner of ~,~ -unsaturated carboxylic acid;

wherein the polymer has a carboxyl to hydroxyl ratio of 1:0.2 to 1:1.8 and an acid number of about 35-150 and a weight average molecular weight of 5,000-80,000; and (2) 10-40~ by weight, based on the welght of the fi~n-formlng constltuent, o~ a water dispersible or water soluble cross-linking resin, preferably an ~069774 alkylated melamine formaldehyde resin ha~ing 1-4 carbon atoms in the alkyl group; and the composltion contains su~ficient water-soluble am~ne to provide ~ pH of about 6-9.

The aforementioned coating composition is usually p~gmented and contains about 0.1-40~o by weight of pigment, but preferably, about 0.1-30~ by weight of pigment. In particular, the composit~on can contain metallic flake pigments such as aluminum ~lake in amounts of 0.1-~.0 by weight. A variety of conventional pigments are utilized in the aforementioned compositions, such as metallic oxide, for example, titanium dioxide, iron oxide, zinc oxide and the like, metal hydroxides, metal powders, chromates, sulphates, carbonates, car~on black, silicate~ talc, phthalocyanine blues and greens, indanthrone pigments and other organic pigments and dyes.

The water-dispersible or water-soluble cross-lin~ing resin in the aforementioned enamel preferably is an all~ylated melamine formaldehyde resin which is com-? patible with the acrylic polymer used in the enamel.Preferably, the enamel contains 65_85% by weight of the acrylic resin and 35-15% by weight of an alkylated melamine formaldehyde resin, and more preferably, about 70% by weight of the acrylic polymer is used in combina-tion with about 30~ by weight of an alkylated melal~ine formaldehyde resin.

Preferably, the alkylated melamine formaldehyde resins used in the aforementioned enamel have 1-4 carbon atoms in the alkyl group and are prepared by conventional techniques in which an alcohol, such as methanol, ethanol, propanol, isopropanol, butanol is reacted with the melamine formaldehyde resin. The melamine formaldehyde resin is reacted with isopropanol and is another use~ul resin.
Also, urea formaldehyde resins can also be used as cross-linking agents, The acryl~c polymer used in the a~orementioned enamel coating composition is partially soluble and partially dispersed in the aqueous medium. Abou-t 30_50~ by weight of the acrylic polymer is dispersed and has a particle size of about 0.01-0.10 micron and preferably about 0.02-o.o6 micron and the residual 50-70% by weight o~ the acrylic polymer is soluble and dissolved in the aqueous medium. To obtain water-solubility and dispersibility, the acrylic polymer preferably has a carboxyl to hydroxyl ratio of 1:0.2 to 1:1.8, which is the molar ratio of carboxyl groups to hydroxyl groups of polymer.

The acrylic polymer used in the aforementioned enamel composition has a uniformity factor of at least 75% and preferably 80-95%. The uniformity factor is the percent o~ polymer in which the constituents are in plus or minus 15% of the average amount given for the polymer. For example, if the average composition of the acrylic polymer is 54% methyl methacrylate, 34%
butyl acrylate, 6% 2-hydroxyethyl acrylate and 6~ acrylic acid; 75% o~ the polymer will be within plus or minus - 15~ of these average values or 54$ plus or minus 8%
methyl methacrylate, 34~ plus or minus 5~0 butyl acrylate, 6 plus or minus 0.9~ 2-hydroxyethyl acrylate and 6 plus or mlnus 0.9~ acrylic acid.

1~69774 The acrylic polymer utilized in the above com-position i~ prepared by a programmed addition of the monomers, polymerization catalysts ~ld solvents. This progra~med addi-tion process is an attempt to form polymer at all stages of the polymerization process ~hich is essentially the same as the predetermined composition~
and results in a polymer composition upon completion of the process ~Ihich has a uni~ormity factor o~ at least 75~. This process allows ~or high percentage conversion of monomers to polymer and also provides a polymer having a relatively uniform molecular weight. These polymers, when used in the novel composition of this invention, provide hlgh quality finishes.

Conventional polymerization processes, such as batch polymerization, commonly used in the art, provide polymers with a wide range of composi~ions and molecular weights which are not suitable for the novel coating compositlon of this invention.

me above programmed addition polymerization process is based on a computer program which uses known polymerization e~uations and activity ratios of monomers to determine the monorner addition rates and ratios and polymer polymerization temperatures and t~nes. This forms a polymer that has a uni~orm composition through-out. The above programmed addition procedure can be based on a computer program which uses a pol~nerization equation in which the polymerization values of the monomers are used. In general, the progra~ned polymeriza-tion pro-cedure comprises an initial charge of monomers and ~069774 solvents which are heated to the reflux temperature in the polymerlzation vessel, then at given intervals monomers and polymer~zation initiator are charged into the vessel while maintaining a re*lux temperature accord-ing to the programmed polymerization procedure~ Throughout the polymeriæation reaction, the polymer being formed has a uniformity factor of at least 75~. In general, the polymerization is conducted at about 75-125C. over a 2-4 hour period to ~orm a polymer that has a weight average molecular weight of about 5,ooo-80,000, and preferably, about 10,000-50,000 determined by gel per-meation chromo-tographyO The polymer has an acid number of about 35-150, pre~erably about 35-80.

Water-miscible solvents are used in the polymerization process to prepare the acrylic polymer, such as isopropanol n-propyl alcohol, 2-ethylhexanol, diacetone alcohol and other alcohols, acetone, acetyl acetone, ethylene glycol monoethylether, ethylene glycol monobutyl ether and ethylene glycol monomethyl ether ~0 acetate. Minor amounts of sol~ents of limited water solubility can be used, such as methylethyl ketone, ethylene glycol monoethyl ether acetate. The novel com-position can contain up to about 20~ by weight of water-miscible solvent but preferably contains 5~15~ by weight of solvent. If desired, the novel composition may be made solvent free.

106~774 About 0.1-4% by welght, based on -the weight o~
the monomer used to prepare the acrylic polymer,o~ the polymerization catalyst is utilized, Typical catalysts are azo-bis-isobutyronitrile, azo-bis~ ~-dimethYl-valeronitrile), benzoyl peroxide, t-butyl peroxypivalate, t~butgl peracetate and the like. Chain transfer agents, ~ - such as lauryl mercaptan are also used~

The acrylic polymer contains 20-60~ by weight of a~hard constituent which can be methyl methacrylate or a mixture of methyl methacrylate and styrene; up to 40% by weight of the polymer can be styrene. The acrylic polymer can contain 5 to 30~ by weight of styrene in combination with 15 to 30% by weight of methyl methacrylate.
Preferably, the polymer contains about 52-57~ by ~Jeight of methyl methacrylate.

The acrylic polymer contains 20-40~ by weight of a soft acrylic constituent which is either an alkyl acrylate that has 2-12 carbon atoms in the alkyl group, an alkyl methacrylate ha~ing 4-12 carbon atoms in the alkyl group or a mixture o~ these two constituents.
P~eferably, the scrylic polymer contains 28 Oo 38S by - lG -.

weight o~ the soft ac~ylic constituent, preferably, an alkyl acrylate having 2-8 carbon atoms in the alkyl group. The following are typical soft acrylic monomers which can be utilized: ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, isobutyl acrylate, - hexyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, nonyl acrylate, laul~l acrylate and the like, butyl meth-acrylate, isobutyl methacrylate, pentyl methacrylate, hexyl methacr~late, octyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, lauryl methacrylate and the like~ Butyl acrylate is the pre~erred so~t.
acrylic constituent since it forms a high quality polymer that has excellent physical propertiesO

The acrylic polymer contains 4-20~o by weight of a hydroxy containing constituent such as a hydroxy alkyl acrylate or a hydroxy alkyl methacrylate or a mix-ture of these two compounds. Preferably, the polymer contains about 5-10~ o~ the hydroxyl contai.ning con~
stituent. These constituents contain 2-4 carbon atoms .

1 ~ .

in the al~yl groups and are, for example, hydroxyethyl acrylate, hydroxybutyl acrylate, hydroxyethyl meth-acrylate, hydroxypropyl methacrylate and hydroxybutyl methacrylate, me acrylic polymer also contains 4-20~ by weight, based on the weight of the acrylic polymer, of an ~ unsaturated carboxylic acid. Typically useful acida are acrylic acid, methacrylic acid, crotonic acid, itaconic acid, propyl acrylic acid, and the like. Prefer-ably, acrylic acid and methacrylic acid are used in amountsof 4-10~ by weight since these acids form high quality polymers.

One preferred acrylic polymer used in this in-vention contains 50-60% by weight of methyl methacrylate, 30-40% by weight of a soft acrylic constituent, prefer-ably butyl acrylate, 5-10~ by weight oP the hydroxy-containing constituent, preferably hydroxyethyl acrylate or hydroxypropyl methacrylate, and 4-12% by weight of acrylic acid, methacrylic acid, or itaconic acid. These preferred acryllc polymers have a weight a~erage molecular weight of about 10,000-50,000, an acid number of about 35-100 and a carboxyl to hydroxyl ratio of about 1:1.03 to 1:1,5.

Another particularly useful acryllc polymer which gives a high quality finish contains about 28-32%
by weight o~ styrene, 22-26~ by weight of methyl meth-acrylate, 30-35% by weight of butyl acrylate, 7-9~ by weight of hydroxy ethyl acrylate and 4-6% by weight o~

acrylic acid ~ld has an acid number of about 30 to 50, a carboxyl to hydroxyl ratio of 1:0.4 to 1:1.5 and a weight average molecular weight oP about 10,00o-5o~ooo.

To form the aqueous dispersion, the acrylic polymer is at least partially neutralized with a water-- soluble amine and then dispersed in water. Typical water-soluble amines that can be used are primary amines~ secondary amines, tertiary amines, polyamines and hydroxyamines, such as ethanol~mine, diethanolamine, triethanolamine, n-methyl-ethanolamine~ N,N-diethylethanolamine, N-aminoethanolamine, N-methyldiethanolamine, monoisopropanol~L;ne, diisopropanol-amineS triisopropanolamine, hydroxyamine, butanolamine, hexanolamine, methyl diethanolamine, N,N-diethylamino-ethylamine, ethylene diamine, diethylene triamine, di-ethylene tetramine, hexamethylene tetramine, triethylamine and the like. The acrylic polymer is usually 50-60~
neutralized and can be 100% neutralized, Neutralization of 50-60% is preferred since this degree of neutralization forms an aqueous dispersion which permits enamel ~ormula-tion at high solids. The pH of the resulting aqueouscoating composltion is generally adjusted to a pH of 6-9, preferably 7.1-7~5.

The novel process of this invention can be utilized to apply the paints over a variety o~ substrates such as metal, wood, glass, steel, iron, plastics, and the like. Preferably~ the novel process is utilized for applying aqueous coatings over primed metal substrates.
Typical al~yd primers and epo~y primers pi~mented with iron oxide, carbon black, titanium dioxide are used.

-- 1~

~069774 The primer can be applied on the metal by electro-deposition or can be applied by conventional spray or dipplng techniques. The process can be utilized to pro-vide coatings directly over galvanized steel to ~orm a durable flnish.

The preferred aqueous acrylic enamel c~n be applied dlrectly over the primed metal su~strates ~th-out the use of an intermediate sealer coat. However, a sealer coat can be used to provide a finish with excellent adhesion and smoothnessO These sealers may be water based or solvent based. One typ cally use~ul sealer composition is disclosed in Rohrbacher, U.S. Patent

3,509,086, issued ~pril 28, 1970.

Finishes applied by the novel process of this invention are characterized by a glossy, smooth and even finish that has a uni~orm appearance, that has excellent water spot resistance, craze resistance, good durability and weatherability and gloss retention and excellent gasoline resistance. These characteristics of the finish applled by the no~el process Or this invention makes the process particularly attractive for applying the exterior finishes on automob~le and truck bodies.

The following Examples illustrate the invention.
The parts and percentages are by weight unless otherwise specl~ied.

~069774 A coating composition is prepared by first forming the following acrylic polymer dispcrsicn:
Parts By Wei~,ht - Portlon 1 Methy] methacrylate monomer 17.080 Butyl acrylate monomer 19.130 2-Ethyl hexyl acrylate monomer 2.720 Acrylic acid monomer 1.150 Isopropanol 6.140 Ethylene glycol rnonobutyl ether9.680 Lauryl mercaptan 0.294 Portion 2 Benzoyl peroxide 0.672 Methylethyl ketone 1.580 Ethylene glycol monomethyl ether acetate 1.580 Ethylene glycol monobutyl ether 2.360 Portion 3 -Methyl methacrylate monomer 24.530 Butyl acrylate mono~.er 11.520 2-Hydroxyethyl acrylate 3.910 Acrylic acid 2.090 Benzoyl peroxlde 0.906 I~opropyl alcohol 3.ooo Ethylene glycol monobutyl ether9.800 Portion 4 Methyl methacrylate monomer 25.720 Butyl acrylate 12.080 2-Hydroxyethyl acrylate monomer4.ogo 3 Acrylic acid monomer 2.200 _ 21 -16~6977~
Parts By ~ei~ht_ Portion 4 (Continued) Benzoyl peroxide 1.248 Isopropanol 4.120 Ethylene glycol monobutyl ether 13.150 Portion 5 Ethyl methacrylate 9.570 Butyl acrylate 4.490 2-Hydroxyethyl acrylate 1~520 Acrylic acid monomer 0.820 10 Benzoyl peroxlde 0.440 Isopropanol 1.460 Ethylene glycol monobutyl ether 4.760 Portion 6 Diethylethanolamine 7.900 Demineral~zed water 101.300 Portion 7 Demineralized water 169.090 Total 492.100 Portion 1 is charged into a reaction veæsel equipped with a stirrer, a heating mantle and a reflux condenser and then heated to the reflux temperature which is about 160C. Portion 2 i8 premixed and then added and then Portion 3 is premixed and added at a uniform rate over a 20-minute period, while maintaining the reaction mixture at ita reflu~ temperature. Portion ~ is premixed and added at a uniform rate over a 60-~nuts period while the reaction mlxture is maintained at its re~lux temperature. Portion 5 i~ premixed ~9~4 and added at a uni~orm rate over a 100-minute period, then the reaction mixtur~ i~ malntained at ~ts reflux tempera-ture ~or an additional 1/2 hour. Portion 6 i~ premixed ana then added to the resction m~ture and then Portion 7 i8 added to the reaction mixture and the reaction mixture i8 cooled to room temperature and filtered to remove any coagulum.
The resultlng polymer dispersion ha~ a 34% polymer solid~ content in which the particle size is about 0.02-0.06 microns. The polymer has a relative visc06ity o~ 1.16 measured at 25C~ in dimethyl formamide at about 0.5% polymer solids and has ~n acid number of about 33 to 35 and a carbo~yl to hydroxyl ratio o~ 1 to 1.5. The polymer ha~ the rollo~ing composition: methyl methacrylate/butyl acrylate/
2-hydroxyethyl acrylate/acrylic acid in a weight ratlo o~
about 54.2/33.1~8.4/4.3 and uni~ormity factor Or about 75-85%-A phthalocyanine blue mill base iB prepared as~ollows:
Part~ By Weight Portion 1 Hexa(metho~ymethyl) melami~e 546 I~opropanol 630 Portion 2 _ Phthalocyanine blue pigment 210 Portion 3 Hexa(methoxymethyl) melamine 285 I~opropanol 426 ~otal 2~97 1~)69774 Portion 1 is mixed with Portion 2 over a 30 minute period and then Portion 3 is added and the constituents are mixed for 30 minutes. The resulting composition is passed through a standard sand mill and ground two passes to provide a uniform mill base.

A blue mill base is prepared as follows:

Parts By Wei~ht Portion 1 Hexa(methoxymethyl) melamine 78 Isopropanol go Portion 2 MONASTRAL* blue pigment 30 Portion 3 Hexa(methoxymethyl) melamine 41 Isopropanol 61 Total 300 Portion 1 is mixe~ with Portion 2 over a 30-minute period and then Portion 3 is added and the constituents are mixed for 30 minutes. The resulting composition is passed through a standard sand-mill and ground for two passes to provide a uniform blue mill base.

A violet mill base is prepared as follows:

* denotes trade mark ~, ,, 377~

Parts By Weight Portion 1 Hexa(methoxymethyl) melamine 13.6 Isopropanol 45.1 Portion 2 MONASTRAL violet pigment 7.o Portion 3 Hexa(methoxymethyl)melamine 7.6 Isopropanol 26.7 Total 100.0 Portions 1 and 2 are blended together and then mixed for 30 minutes and then Portion 3 is added and the composition is mixed for an additional 15 minutes. The resulting composition is then ground two passes in a standard sand-grinding mill to form a uniform mill base.

A phthalocyanine green-yellow mill is formed as ~ollows:

Parts By Weight Portion 1 Hexa~methoxymethyl) melamine 78 Isopropanol go Portion 2 Phthalocyanine green-yellow pigment 30 Portion 3 Hexa(methoxymethyl) melamine 41 Isopropanol 61 Total 300 .~

Portion 1 is blended with Portion 2 in a mlxing vessel for 30 minutes and then Portion 3 ls added and blended to an additional 15 mlnutes. The resultlng composition is then ground two passes in a standard sand-grinding mill to form a uniform mill base.
An aluminum flake mill base is prepared as ~ollo~s:
Parts By Weight 10 Aluminum flake 1.71 Hexa(methox~nethyl) melamine 5-75 Isopropanol 11.05 - - Total 18.51 The above constituents are thoroughly blended together for 30 minutes to form a uniform dispersion.

A paint composition is prepared by blending together the followlng ingredients: -Parts By 20 Portion l Weight Phthalocyanine Blue mill base (prepared 2.bove) 6.35 Blue mill base (prepared above) 0.20 Violet mill base (prepared above) 0,70 Phthalocyanine green-yellow mill base (prepared above) 0.45 Aluminum flake mill base (prepared above) 11.05 Portion 2 = ~
Hexa(methoxymethyl) melamine 20.40 Portion 3 Acrylic polymer dlspersion (prepared above) 272.40 - 2~, -parts By Weight Portlon 4 Deionized ~ater 18.00 Port~on 5 Butyl acrylate/acrylic acid copolymer solution - (80% polymer solid~ in alcohol o~ an 85/15 butyl acrylate/acrylic acid copolymer) 3~30 Silicone anti-cratering agent solut~on (10% low molecul~r weight ~ilicon re~in ln water) 3.35 10 Deionized water 31.00 Total367.10 Portion 1 i~ charged in a mi~1ng veæsel and thoroughl~ blended together and then Portion 2 i~ added then blended with Portion 1 and Portion~ 3~ 4 and 5 are added consecutively w~th blending after e&ch addition. me resulting composition haæ a 30 second vi~cosity uæing a No. 2 Fi~her cup and a total solids content o~ 28.1%.
A Bink~ 62-~ spray gun is used with a 63 PB cap, palnt ~low rate 16 ounces per minute, atomizing air pre~sure 70 p~. All spraying i8 done wlth an Eclipæe automatic machine with an index speed æet at 900 inches per minute travel. Four coats of paint are applied wlth two spra~ passes for each coat and with a two minute ~la~h time between coatæ. The gun ls po~itloned 14 inches ~rom the panel. The panel~ are 4" x 6" of phosphatized steel primed with a 1.5 mil iron oxide pigmented alkyd resin primer. After application~ each of the panels is baked for 15 minutes at 85C. and then 45 minutes at 150C.

~069774 Panels are sprayed under the following conditions:

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- 29 _ 106977~

A coating composition is prepared as follows:

Parts By Weight Portion 1 ~ . _ Acrylic polymer dispersion (solids - content of 70~ and the polymer has a relative viscosity of about 1.09 measured at 25 in dimethyl formamide and about 0.5% polymer solids and an acid-number of about 47 and a carboxyl to hydroxyl ratio of 1 to o.58 and is of the follo~ing composition:
methyl methacrylate/butyl acrylate/
2-hydroxyethyl acrylate/acrylic acid in a weight ratio of about 54/34.2/
5.7/6.1 and has a uniformity factor of about 75-85~) 200 - Portion 2 Diethylaminoethanolamine 14 Portion 3 Deionized water 318 Portion 4 Hexa(methoxymethyl)melamine) 60 Total 592 Portion 1 is charged into a mixing vessel and then Portion 2 is added and blended with Portion 1. Portion 3 is then slowly added with constant agitation. After Portion 3 is added, all of Portion 4 is then added and mixed in. The resulting coating composition has a solids content of about 33~ and a spray viscosity of about 30 seconds using a No. 2 Parlin cup.

106~774 The above prepared coating composition is applied as in Example 1 using the ~dentical spraying and baking procedures and the same primed steel panels under the same ambient air and controlled air conditions. The results of the application are very similar to the results of Exarnple 1. The panels sprayed under uncontolled conditions exhibited running and sagging.

A coating composition is prepared by blending the following ingredients:
Parts By Weight Portion 1 Acrylic polymer dispersion (solids content of 71% and the polymer has a relative viscosity of 1.096 measured at 25C. in dimethyl formamide at about 0.5% polymer solids, an acid number of about 93 to 94 and a carboxyl to hydroxyl ratio of 1 to 0.31 and is of the following composition: methyl methacrylate/
butyl acrylate/hydroxyethyl acrylate/
acrylic acid - 53.8/28.2/6.0/12.0 and has a uniformity factor of about 75-85%.) 200 Portion 2 Diethylaminoethanolamine 15 Portion 3 Deionized water 318 Portion 4 ~exa(methoxymethyl) melamine 60 Total 593 3~

Portion 1 is charged into a mixing vessel and then Portion 2 is added with constant agitation and then Portion 3 is slowly added to the reaction mixture and after Portion 3 has been added, Portion 4 is added and blended with the reaction mixture. The resulting composition has a 33% polymer solids content and a spray viscosity of 30 seconds measured in a No. 2 Parlin cup.
The above prepared coating composition is applied as in Example 1 using the identical spraying and baking procedures and the same primed steel panels under the same ambient air and controlled air conditions. The results of the application are very similar to the results o~ Example 1. The panels sprayed under controlled conditions have a good appearance while the panel sprayed under uncontrolled conditions exhibited running and sagging.

A paint composition is prepared by blending together the following ingredients:

Parts By Weight Portion 1 Mill base (described in Example 1) 6-35 MONASTRAL blue mill base (prepared in Example 1) 0.20 MONASTRAL violet mill base (prepared in Example 1) 0.70 Phthalocyanine green-yellow mill base (prepared in Example 1) 0.45 Aluminum flake mill base (prepared in Example 1) 11.05 ,~, , .

1069774 Parts By Wei~ht Po_tion 2 Hexa(methoxymethyl) melamine 20.40 Portion 3 .
- Acrylic polymer dispersion (polymer solids content of 26~ by weight and the polymer has a relative viscosity of about 1.15 measured at 25C. in di-methyl formamide solvent at about 0.5~polymer solids) an acid number of about 46 to 47 and a carboxyl to hydroxyl ratio of 1 to 0. 62 and the polyrner is of the composition: methy]. methacrylate/
butyl acrylate/2-hydroxyethy]. acrylate/
acrylic acid in a weight ratio of about 54/34/6/6 and has a uni-formity factor of about 75-85%) 272.40 Portion 4 Deionized water 18.00 Portion 5 Butyl acrylate/acrylic acid copolymer solution (described in Example 1) 3.30 Anti-cratering solution (10~ silîcone solution) 3. 35 Deionized water 31.00 Total 367.20 Portion 1 is charged into a mixing vessel and thoroughly blended together and then Portion 2 is added and blended and then Portion 3 is added and the consti-tuents are thoroughly blended together. Portion 4 is added and blended with the mixture and then Portion 5 is added and thoroughly blended with.the mixture. The resutling paint composition has a 28~ solids content and a spray viscosity of 31 seconds using a No. 2 Zahn cup.

- 33 ~

The above prepared coating composition is applied as in Example 1 using the identica] spraying and baking procedures and the same primed steel panels under the same ambient air and controlled air conditions. The results of the application are very similar to the results o~ Example 1. The panels sprayed under controlled conditions have a good appearance while the panel sprayed under uncontrolled conditions exhibited running and sagging.

_ 3~ _

Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for applying an aqueous dispersion or solution paint to a substrate by air-spraying the paint with a spray gun utilizing atomizing air at a flow rate of 5-40 cubic feet per minute and at an air pressure of 30-95 pounds per square inch at the spray gun and a paint flow rate of 5-30 ounces per minute which forms an air atomized paint spray which is applied to the substrate and baked at about 75 to 200°C. to form a uniform finish on the substrate, the improvement in use therewith comprising spraying the atomized paint in an air atmosphere that is controlled by the addition of steam or water at a temperature of 15-40°C. and at such a relative humidity that the air has a driving force value in the range of 0.002 to 0.004 humidity units, in pounds of water per pound of dry air, which is constantly maintained at a set value within this range, thereby providing an atomsphere for initially drying the aqueous paint during the spraying at a uniform rate to provide a finish that has a uniform appearance after being baked;

wherein the paint is an aqueous thermosetting acrylic enamel in which the film-forming constituents of the enamel comprise:

(1) an acrylic polymer selected from the group consisting of (A) a polymer of (a) at least one of styrene and methyl methacrylate, (b) at least one of an alkyl acrylate and an alkyl methacrylate, where the alkyl group has 2-12 carbon atoms, and (c) an .alpha.,.beta. unsaturated carboxylic acid, and (B) a polymer of (a), (b) and (c) and at least one of a hydroxy alkyl acrylate and a hydroxy methacrylate;
and (2) a water soluble or water-dispersible cross-linking agent.

2. The process of Claim 1 in which the air has a driving force value of 0.003 humidity units.

3. The process of Claim 1 in which the acrylic enamel comprises 10 to 60% by weight of film-forming con-stituents and correspondingly 90 to 40% by weight of water and up to 20% by weight of a solvent for the film-forming constituents; wherein the film-forming constituents consist essentially of (1) 60-90% by weight, based on the weight of the film-forming constituents, of an acrylic polymer that consists essentially of (a) 20-60% by weight, based on the weight of the acrylic polymer, of a hard constituent which is selected from the group consisting of acrylate and styrene, and mixtures thereof;

(b) 20-40% by weight, based on the weight of the acrylic polymer, of a soft constituent that is selected from the group consisting of an alkyl acrylate having 2-12 carbon atoms in the alkyl group and an alkyl methacrylate having 4-12 carbon atoms in the alkyl group, and mixtures thereof;

(c) 4 to 20% by weight, based on the weight of the acrylic polymer, of a hydroxy containing constituent which is selected from the group consisting of a hydroxy alkyl methacrylate and a hydroxy alkyl acrylate, and mixtures thereof, in which the alkyl groups have 2-4 carbon atoms;

(d) 4-20% by weight, based on the weight of the acrylic polymer, of an .alpha.,.beta.-ethyleni-cally unsaturated carboxylic acid;

(2) 10-40% by weight, based on the weight of the film-forming constituents, of a water-dispersible alkylated melamine formaldehyde resin having 1-4 carbon atoms in the alkyl groups;

and the composition contains sufficient water-soluble amine to provide a pH of about 6-10.

4. The process of Claim 3 in which the acrylic polymer of the enamel consists essentially of 50-60%
by weight of methyl methacrylate, 30-40% by weight of butyl acrylate, 5-10% by weight of hydroxyethyl acrylate, and 4-12% by weight of acrylic acid and the polymer has an acid number of about 35-100 and a carboxyl to hydroxyl ratio of about 1:0.3 to 1:1.5, then the acrylic polymer has a weight average molecular weight of about 10,000-50,000.

5. The process of Claim 3 in which the acrylic polymer of the enamel consists essentially of 28-32%
by weight of styrene, 22-26% by weight of methyl meth-acrylate, 30-35% by weight of butyl acrylate, 7-9% by weight of hydroxyethyl acrylate, 4-6% by weight of acrylic acid and has an acid number of about 30-50 and a carboxyl to hydroxyl ratio of 1:0.4 to 1:1.5 and the polymer has a weight average molecular weight of about 10,000-50,000.

6. The process of Claim 1 in which the atomizing air is heated to 40°C. to 150°C.

7. me process of Claim 1 in which the spray gun is a hand spray gun.

8. The process of Claim 1 in which the spray gun is an automatic spray gun.

9. The process of Claim 1 in which the sub-strate is steel.

10. The process of Claim 1 in which the substrate is a steel substrate coated with a primer composition.